Differential pressure forming machinery for thermoplastic synthetic plastic sheets, and wherein the sheets are individually clamped in carriages and move in a continuous path from a loading station (where the sheets are individually loaded to a sheet clamping carriage), to a heating station (where the sheets are heated to a forming temperature at which they may be formed) to a forming or molding station (wherein the sheets are deformed by way of direct compression or differential pressure forces, or a combination of these, to the configuration of molds) and thence are indexed to an unloading station, and such machinery is disclosed for example in the present assignee's Brown Pat. No. 3,583,036. As this prior U.S. patent indicates, the heating station may be made up of two sets of upper and lower heating mechanisms arranged respectively above and below the path of travel of the sheets and a typical loading-unloading station may comprise a location at which the sheet which has been formed is unloaded and a circumferentially adjacent location at which a new sheet is reloaded. Differential pressure plastics forming machinery of this character conventionally employs radiant heaters which are located a predetermined distance away from the path of travel of the sheet at a heating station which may comprise one or more heating locations and sets of heaters, and normally the sheets sagged after being heated, as the heated portions became deformable and lost mechanical strength.
Recently, it has been determined that certain superplastic metal alloys can be formed much like plastics under very small forces and at temperatures in the neighborhood of 500.degree. F. This property of superplasticity, which allows particular alloys such as zinc alloys (which may comprise 78% zinc and 22% aluminum) to be formed, is described in U.S. Pats. Nos. 3,340,101, 3,567,524, and 3,420,717. Forming processes for such superplastic alloys similarly include vacuum forming and compression forming.
One of the prime objects of the present invention is to provide contact heater mechanism which is particularly useful for heating certain thermoplastic synthetic plastics and superplastic metal alloys to a forming termperature. For example, it has been found that the apparatus which will be described transfers heat to the high moisture content plastic sheets or webs in a manner which does not cause the moisture to bubble out of the plastic or permit the plastic sheet to sag. Formerly, problems have been encountered with plastics such as certain polycarbonates and polystyrenes which attract moisture and build up a relatively high moisture content, and of course, sagging has traditionally been a well-recognized and accepted condition which is difficult to control. In present practice, separate ovens must operate at temperatures in the neighborhood of 1100.degree. F. in order to accomplish pre-drying of such material before it is then cooled and loaded to conventional forming machinery wherein reheating occurs and forming is carried out at sheet-forming temperatures in the range of 280.degree. - 320.degree. F. This not only is costly in terms of power consumption, it also greatly increases fire hazards in the area of operation.
At the same time, it has been found that such contact heater mechanisms are extremely well suited to the heating of superplastic metallic alloys to forming temperatures in the neighborhood of 500.degree. F., without creating sag or other problems.
It is a principal object of the present invention to provide highly reliable and efficient apparatus which is capable of use to heat both plastics and superplastic metals to forming temperatures in a manner which avoids many of the problems presently encountered with other heating methods.
Another important object of the invention is to provide mechanism which is constructed to permit thermal expansion of some of the parts relative to the others.
Still another object of the invention is to provide a construction employing a sandwiched heating mechanism wherein electrical resistance heating elements are enclosed within the confines of sandwiched heater plates mounting contactor plates which are moved toward and away from heat transfer relationship with the sheet to be heated to forming temperature.
Still another object of the invention is to provide a construction of the character mentioned wherein changeover costs are reduced and the contactor plates can be readily removed and changed to permit contactor plates of different size or shape to be mounted in position to heat sheets of varying size and shape.
Another object of the invention is to provide contact heater mechanism utilizing a multiplicity of electrical resistance heater elements which can be individually activated or deactivated to provide a zoned heat control which can selectively impart heat to particular areas of the contactor plates.
Another object of the invention is to provide contact heater apparatus of the character mentioned wherein strip heaters are employed, and in which they are arranged so that the creation of large cold spots is avoided.
Other objects and advantages of the invention will be pointed out specifically or will become apparent from the following description when it is considered in conjunction with the appended claims.